In the “12th Five Year Plan” period of China, the development strategy of the coal industry in China is “Controlling in the Eastern China Region, Stabilizing in the Central China Region, and Vigorously Developing in the Western China Region”. In the Western China region, the Country will vigorously propel the construction of large-size coal bases, especially the construction of a number of 10-million ton large-size modern coal mines. The mine area in the Western China region have abundant coal resources, the coal seams have a high risk of spontaneous ignition and are buried under shallow depths (usually not deeper than 200 m), the spacing between coal seams is smaller and the roof bedrocks are thin, a fully mechanized top coal caving technique is mainly used at the working faces, large-area surface collapse and fissures may occur owing to the mining disturbance, and a large quantity of air leakage passages may be formed between the surface to the goaf; consequently, air leakage from the surface becomes severe, and spontaneous ignition of coal in the goaf frequently occurs; meanwhile, the goafs are connected with each other to form a large-area region, which increases the difficulties in prevention and control of spontaneous ignition of coal in the goafs, which severely impacts safe and efficient mining in the coal mines, and leads to severe economic losses and social influences. According to incomplete statistics, more than 200 spontaneous coal ignition accidents which resulting in closing working faces happened in western mine area in the last 10 years, resulting in direct economic losses equivalent to more than RMB 10 billion.
In worldwide, fire prevention and extinguishment techniques such as grouting, nitrogen injection, foam injection, retardant spraying, gel and composite colloid injection, etc., are usually used to prevent and control spontaneous ignition of coal in the coal mines goafs. With the grouting technique, the grout only flows towards lower lying area in the goaf; consequently, the coverage area is small, the grout cannot be accumulated to higher parts, and a “grooved” phenomenon may occur easily; meanwhile, the working faces in western coal mines have long length, high mining intensity and quick advancing rate, and it is unsuitable to set up a permanent surface grouting system in the coal mines; furthermore, owing to the fact that the western mine areas are short of water and soil, it is difficult to implement conventional grouting. The nitrogen injection technique has been widely employed in many mine areas in the last 10 years, owing to the characteristics of nitrogen, such as inerting burning area and wide diffusion area, etc.; however, the nitrogen tends to escape with the air leakage, and the fire extinguishment and cooling ability of nitrogen are weak; meanwhile, owing to the fact that the coal seams in the western mine area are buried under shallow depths, and the goafs are inter-connected to large area, and communicate with the surface fissures, it is difficult to create closed spaces in the goafs; therefore, conventional nitrogen injection cannot attain the purpose of inerting the goafs quickly. With the foam injection technique, though foam injection overcomes some drawbacks of grouting and nitrogen injection and the foams can be accumulated to higher parts, the flow and diffusion range of foams that are injected at high flow rate and have strong diffusibility is still limited in the large-area goaf with a small slope angle; consequently, the foams still cannot completely and effectively cover the float coal and air-leaking fissures in the large-area goaf. With the retardant spraying technique, the retardant may corrode the downhole equipments and harm physical and mental health of the workers, and the fire prevention and extinguishment effect is not ideal. With the gel and composite colloid injection technique, the gel or colloidal mud has a small flow amount but a high cost, and the diffusion area is small; therefore, gel and composite colloid injection is unsuitable for prevention and control of spontaneous ignition of coal in a large-area goaf.